1. Field of the Invention
The present invention relates to a method and apparatus for producing a synthetic resin film. Particularly it relates to a method and apparatus for producing a synthetic resin film in which a tenter is used, which tenter stretches a continuously supplied film in the transverse direction or the transverse and longitudinal directions while the film travels under being clamped at both lateral edges thereof by moving clamp means, wherein improvement was achieved in clamping positions in the transverse direction and/or in the longitudinal direction of the film. The improvement brings reducing abnormal deformation or breaking of the film in the tenter and promoting efficiency in production of films.
2. Description of the Prior Art
Tenters in which a continuously supplied film is stretched in the transverse direction or the transverse and longitudinal directions while the film clamped at both lateral edges thereof by moving clamp means is moved therein have heretofore been used. Generally, the former tenter is called a transverse stretching machine and the latter tenter is called a simultaneous biaxial stretching. Such tenters are described, for example, in Japanese Patent Laid-Open (Kokai) No. HEI 6-191700 and Japanese Patent Publication (Kokoku) No. SHO 57-57260 (U.S. Pat. Nos. 4,173,611 and 4,210,987).
The former document discloses a means for adjusting positions of clamping means in the transverse direction based on signals obtained from detection of positions of both lateral edges of a film running toward a tenter.
The latter document discloses a tenter having a route (first route) for letting a continuously supplied film continuously run toward clamp means of a tenter and a route (second route) for letting a junk film continuously run toward a junk take-over device, and also having a means for selecting either of the routes,: i.e., a tenter having a junk film take-over device.
When a film is produced using a tenter having a junk film take-over device, the film running route is changed from the second route to the first route, and both longitudinal edges of the film arriving at the tenter are clamped by the clamp means and stretching in transverse direction is commenced. However, in this case, a phenomenon is observed, that the film is wrinkled and it is difficult to maintain the continuous transverse stretching. This phenomenon may be improved to some extent by adjusting the positions of the clamp means by detecting the positions of both lateral edges of the film running toward the tenter disclosed in the former document, but the improvement is not sufficient.
On the other hand, the conventional tenters are designed to ensure that the clamp means positively clamp the tip of the film when the tip of the supplied film is accepted by the clamp means. In these conventional tenters, the film is likely to be broken at the center portion in the transverse direction at the tip of the clamped film, and a phenomenon can be observed, that the breaking occurred at the tip propagates in the longitudinal direction of the film.
Each of the above phenomena becomes more and more conspicuous where the width of a film [is become] becomes broad or the speed of film fed to a tenter becomes high to promote efficiency of production of film.
The object of the present invention is to provide a method and apparatus for producing a film improved in the above problems of the conventional tenters.
The method for producing a synthetic resin film of the present invention to achieve the above object is as follows.
A method for producing a synthetic resin film: comprising a film supplying step for supplying an original film, a film tentering step for clamping both longitudinal edges of the film supplied from the film supplying step by a pair of continuously moving clamp means provided along both side edges of the film and for stretching the clamped film in the transverse direction of the film, a film recovering step for taking over the film supplied from the film supplying step, without feeding it to the film tentering step, and a film running direction changing step for cutting the film running toward the film recovering step and feeding the tip of the film to the film tentering step, and further comprising detecting the side edge position of the film fed toward the film recovering step and changing, the positions of the pair of clamp means in the transverse direction of the film based on a signal obtained through the detecting.
In the present invention, a tentering step or tenter means a step or apparatus having a function of stretching a supplied undrawn or drawn film at least in transverse (crosswise) direction of the film.
In the process for producing a synthetic resin film, it is preferable that the distance between a portion on the film fed to the film tentering step clamped at first by the clamp means and the tip of the film in the longitudinal direction of the film is not less than 5 mm and not more than 1,000 mm.
the part of the film between a portion on the film fed to the film tentering step clamped at first by the clamp means in the longitudinal direction is called the clamp-free portion, and the distance of the clamp-free portion is called the clamp-free length (CFL).
The idea of clamping the film remaining the clamp-free portion can also be applied to a film production process free from the film recovering step, and it reduces the breaking of the film likely to be caused at the tip of the clamped film, thus improving the film production efficiency is also obtained in the process.